Geology in space

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Tis’ the reason for the season

Happy Solstice everyone! Today in the northern hemisphere is the shortest day (longest in the southern hemisphere).

If you imagine a stick through the Earth around which the planet spins on a daily basis this is the Earth’s axis. If this axis was vertical, then the lengths of day would not change, throughout the year, however it actually lies out at an angle of 23 degrees from the vertical. At different points in its orbit the north will be pointing either in the direction of the sun or further away. Today the northern hemisphere is pointing directly away from the sun. This means that in the northern hemisphere the days are much shorter and the nights longer due to spending more time facing away from the Sun than towards it. In addition due to the curvature of the Earth the beams of light hitting the surface is more spread out towards the poles than the equator and so the amount of incoming energy spreads out, these two mean that the climate gets colder during the winter. What about other planets, are there seasons and how are they manifested?

The north pole of Mercury, some of the craters are permanently in shadow (NASA/ John Hopkins/Carnegie Institution)

There are huge temperature variations on Mercury related to the eccentricity of its orbit (how elliptical it is rather than circular) linked with a 3:2 ratio of years to days but these do not cause temperature changes in latitude.

Venus has a tilt of 177 degrees, what this means is that it is completely flipped over when you look at its rotation (it spins clockwise whilst the other planets spin anticlockwise)

That being said it means that the axis is only about 3 degrees off the vertical and with the very efficient heat transport in the dense atmosphere the temperature is fairly constant over the whole globe and that Venus doesn’t have a strong seasonal changes.

With a similar axial tilt to Earth (25 degrees) Mars also has seasons, which are about twice as long as on Earth (due to the longer year). This leads to growing and shrinking of carbon dioxide ice caps and temperature changes just as on Earth. Intriguingly images from the Mars reconnaissance orbiter have shown linear features called recurring slope lineae forming on crater edges, these features grow during the warmest months then disappear during the coolest month. They are thought to be formed from brines (very salty water) which melt and run down the slope, they do not appear in the winters due to it being too cold for these brines to melt, although the source of the water is not currently certain.

Saturn has seasons which last around 7 Earth years, changes in cloud composition and occurs during this transition and there are increased storms during spring

Uranus is lying on it side, meaning that the axial tilt is just of the equator which means that the poles experience 42 years of day light followed by 42 of darkness, the change in temperature between the side facing the sun and the side facing away the sun probably has an effect on its climate however as it has only been briefly visited by the Voyager 2 probe little is known about the long term seasonality ice giant.

Finally Neptune has a similar axial tilt to Earth of 28 degrees, at the moment a lack of observational evidence makes it difficult to say if it has any strong seasonal effects although an increase in cloud cover has been noticed by Hubble as it transitions into a 40 year long summer.